In the manufacture of glass fibers, molten streams of glass are attenuated from containers known in the art as bushings through a plurality of holes or orifices and the resulting filaments are attenuated and gathered into strand form. In some instances, the filaments are chopped into discrete lengths as they are being attenuated. The containers used are typically constructed of precious metals such as platinum or alloys thereof, platinum-rhodium alloys being a typical alloy used.
The bushings used to form fibers are typically rectangular containers having sidewalls and an open top and a bottom. Tips or nozzles which communicate with orifices in the bushing bottom are provided for glass flow and by properly sizing the tips and adjusting the pull rates on the molten glass which issues through the tips, the diameter of the filaments or fibers formed is determined at a given glass temperature. Molten glass is delivered to the open top of the bushing from the forehearth. A detailed description of the fiber drawing process is contained in chapter 5 of the book "The Manufacturing Technology of Continuous Glass Fibers", K. L. Lowenstein, Elsevier Scientific Publishing Co., New York, 1973.
In a fiber forming process, many bushings are employed for a single melting furnace and the molten glass from the furnace is typically fed along a forehearth which is provided with a plurality of openings to which the bushings can be operatively connected so that glass can flow from the forehearth into each of several bushing positions. FIG. 3 of U.S. Pat. No. 3,837,823 shows a diagram of a typical melter-forehearth-bushing arrangement. Similar schemes are shown on pages 61-63 of the Lowenstein book cited herein above. The bushings are connected to the forehearth by mounting the bushing in the frame and bolting the frame to refractory blocks positioned in the forehearth openings. FIG. 2 of U.S. Pat. No. 3,837,823 and FIG. V-1 on page 91 of the Lowenstein book above cited, show typical arrangements of this connection.
When a new furnace is started, it is necessary to employ in the forehearth openings a modified bushing called a drain or starter bushing. These devices are used to remove molten glass from the furnaces and forehearths when the furnaces are started up. Using these bushings, the first made glass in the melter, forehearth and refiner is removed to dispose of subquality glass, refractory fragments from the refractory lining used in the melters, forehearths and refiners and any other debris that may be present. Thus, the furnace or melter and its associated refiners and forehearths are thoroughly cleaned of debris using molten glass flowing through the drain bushings. This procedure of using drain bushings in a large modern fiber glass making direct melt system continues until consistent quality glass is being made and takes normally from about 1 to about 5 days. Once quality glass is being produced, the drain bushings are replaced by production bushings and products are then produced on the winders or choppers used under the production bushings to attenuate the fibers issuing from the production bushings.
The drain bushings used to start up a furnace are typically similar to the production bushings except for the bottom or faceplate of the bushing. The bottom of a production bushing usually contains rows of tips associated with orifices in the bottom. The number of tips can range from 400 to 6,000 or more in a modern plant, 800 tips to 2,000 tips being typical in most operations. The drain bushings on the other hand, will have usually one to five holes, typically 2, in the bottom which are of a diameter much larger than the production bushing orifice. A drain bushing, for example, may contain two holes, usually one on each side of the bushing near the electrical input thereto and ranging in a diameter of from about 0.187 to about 0.50 inches. Production bushing orifices on the other hand are typically from about 0.040 to about 0.110 inches in diameter depending on the size of the glass filaments desired. Both bushings, however, have a well or a containment area for holding molten glass formed by the sides of the bushing and the bottom and bushing connectors or bushing ears are positioned on the sidewalls so that the bushing can be connected to an electrical supply used to maintain glass in the bushing in the molten state. The size of the bushing well formed by the bushing sides and associated ears or connectors all contribute to the overall weight of the bushing. Thus, a drain bushing will range in weight depending on its overall dimensions from 900 to 1300 grams. As will be readily appreciated, in a large modern direct melt plant the complement of drain and production bushings required at the start up of a new furnace means dedicating a large quantity of precious metal to that tank or furnace during the start up procedure. The dedication of such large quantities of precious metal is costly and the fabrication of the drain bushings is also costly as is the handling of the metal after it is used, i.e., remelting and refabricating the metal into other useful forms. Further, since the normal drain bushings have a well area for molten glass they are similar to a production bushing which means that their removal from the forehearth opening is as difficult as it is with a production bushing. Thus, the glass in the bushing and the forehearth opening above the bushing must be frozen by cooling before removal and a considerable amount of time and effort is expended chipping the solidified glass in the bushing opening before the bushing can be removed.
Attempts have been made in the art to utilize other materials such as nickel for drain bushings with mixed success. These nickel bushings operate close to the nickel metal softening point and the electrical characteristics of nickel are inferior to those of the platinum alloys normally used by fiber glass manufacturers. These nickel bushings are also subject to severe wear and must be watched closely and replaced frequently while the start up procedure is being conducted. These bushings also resemble a production bushing and having a glass well are also difficult to remove from the forehearth.
Thus, a need exists to provide a safe, effective drain bushing which can operate to drain the initial glass made in a melting furnace and which will substantially reduce the quantity of precious metal heretofore required of conventional drain bushings. In accordance with the instant invention, a bushing is provided which supplies that need and other benefits as well.